Figure 2: Temperature Biosensor Housing Drawn in SolidWorks (Front View)

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Hilary Smith
6 years ago
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David Knoff Senior Design Weekly Report 3/15/2013 Overview and Completed Work: This week I focused on drawing the temperature biosensor housing in SolidWorks and getting the fall detection

00 in dimensions because it will house the PCB board snug but have some excess room for a thin wall and minimal spacing between.

1 David Knoff Senior Design Weekly Report 3/15/2013 Overview and Completed Work: This week I focused on drawing the temperature biosensor housing in SolidWorks and getting the fall detection biosensor working with functionality. Figure 1 and Figure 2 show different views of the temperature biosensor housing. I chose 2.00 in X 2.00 in dimensions because it will house the PCB board snug but have some excess room for a thin wall and minimal spacing between. I rounded the corners to reduce the risk of puncturing or harming the patient in any way. I chose a height of 0.25 in to accommodate the height of soldered elements. Figure 1: Temperature Biosensor housing Drawn in SolidWorks (Top View) Figure 2: Temperature Biosensor Housing Drawn in SolidWorks (Front View)

2 I also focused on programming the fall detection biosensor in C programming language. Using the ADXL345 datasheet and other user s code guidance on several open sharing websites/forums, I wrote the program to transport data through SPI communication from the breakout board to the Arduino Due master controller. Figure 3 shows the current code, excluding programmed interrupts which will provide alarm functionality. //Add the SPI library so we can communicate with the ADXL345 sensor #include <SPI.h> //Assign the Chip Select signal to pin 10. int CS=10; //This is a list of some of the registers available on the ADXL345. //To learn more about these and the rest of the registers on the ADXL345, read the datasheet! char POWER_CTL = 0x2D; //Power Control Register char DATA_FORMAT = 0x31; char DATAX0 = 0x32; //X-Axis Data 0 char DATAX1 = 0x33; //X-Axis Data 1 char DATAY0 = 0x34; //Y-Axis Data 0 char DATAY1 = 0x35; //Y-Axis Data 1 char DATAZ0 = 0x36; //Z-Axis Data 0 char DATAZ1 = 0x37; //Z-Axis Data 1 //This buffer will hold values read from the ADXL345 registers. char values[10]; //These variables will be used to hold the x,y and z axis accelerometer values. int x,y,z; double xg,yg,zg; void setup(){ //Initiate an SPI communication instance. SPI.begin(); //Configure the SPI connection for the ADXL345. SPI.setDataMode(SPI_MODE3); //Create a serial connection to display the data on the terminal. Serial.begin(9600); //Set up the Chip Select pin to be an output from the Arduino. pinmode(cs, OUTPUT); //Before communication starts, the Chip Select pin needs to be set high. digitalwrite(cs, HIGH); //Put the ADXL345 into +/- 4G range by writing the value 0x01 to the DATA_FORMAT register. writeregister(data_format, 0x01); //Put the ADXL345 into Measurement Mode by writing 0x08 to the POWER_CTL register. writeregister(power_ctl, 0x08); //Measurement mode void loop(){ //Reading 6 bytes of data starting at register DATAX0 will retrieve the x,y and z acceleration values from the ADXL345. //The results of the read operation will get stored to the values[] buffer. readregister(datax0, 6, values);

3 //The ADXL345 gives 10-bit acceleration values, but they are stored as bytes (8-bits). To get the full value, two bytes must be combined for each axis. //The X value is stored in values[0] and values[1]. x = ((int)values[1]<<8) (int)values[0]; //The Y value is stored in values[2] and values[3]. y = ((int)values[3]<<8) (int)values[2]; //The Z value is stored in values[4] and values[5]. z = ((int)values[5]<<8) (int)values[4]; //Print the results to the terminal. Serial.print(x, DEC); Serial.print(','); Serial.print(y, DEC); Serial.print(','); Serial.println(z, DEC); delay(10); //This function will write a value to a register on the ADXL345. //Parameters: // char registeraddress - The register to write a value to // char value - The value to be written to the specified register. void writeregister(char registeraddress, char value){ //Set Chip Select pin low to signal the beginning of an SPI packet. digitalwrite(cs, LOW); //Transfer the register address over SPI. SPI.transfer(registerAddress); //Transfer the desired register value over SPI. SPI.transfer(value); //Set the Chip Select pin high to signal the end of an SPI packet. digitalwrite(cs, HIGH); //This function will read a certain number of registers starting from a specified address and store their values in a buffer. //Parameters: // char registeraddress - The register addresse to start the read sequence from. // int numbytes - The number of registers that should be read. // char * values - A pointer to a buffer where the results of the operation should be stored. void readregister(char registeraddress, int numbytes, char * values){ //Since we're performing a read operation, the most significant bit of the register address should be set. char address = 0x80 registeraddress; //If we're doing a multi-byte read, bit 6 needs to be set as well. if(numbytes > 1)address = address 0x40; //Set the Chip select pin low to start an SPI packet. digitalwrite(cs, LOW); //Transfer the starting register address that needs to be read. SPI.transfer(address); //Continue to read registers until we've read the number specified, storing the results to the input buffer. for(int SPI.transfer(0x00); //Set the Chips Select pin high to end the SPI packet. digitalwrite(cs, HIGH);

4 void loop() { //Convert the accelerometer value to G's. //With 10 bits measuring over a +/-4g range we can find how to convert by using the equation: // Gs = Measurement Value * (G-range/(2^10)) or Gs = Measurement Value * (8/1024) xg = x * ; yg = y * ; zg = z * ; //print data for x, y, and z axes Serial.println('X-Axis'); Serial.print((float)xg); Serial.print("g,"); Serial.println('Y-Axis'); Serial.print((float)yg); Serial.print("g,"); Serial.println('Z-Axis'); Serial.print((float)zg); Serial.print("g,"); Figure 3: Arduino Due Program for ADXL345 Breakout Board Equation 1 shows the scale needed to convert the ADXL345 output acceleration values to export data in g s. This scale is for the +/- 4g mode on the ADXL345. I chose this mode because for our application +/- 4gs is more than enough acceleration detection sensitivity than we need. In addition to coding the ADXL345 for access by the Arduino master controller, it is necessary to calibrate the ADXL345 chip to orient each axis by finding the value the accelerometer outputs when each axis equals zero gravity. This can be done manually by orienting each axis at zero gravity and averaging 10 values which the accelerometer outputs. However, it is more accurate to calibrate each axis at one time to ensure proper orientation of each axis. Figure 4 shows the code used for calibrating the accelerometer while it remains still on a flat, horizontal surface. This position orients the x and y axes at zero gravity, however the z axis is compensated for by subtracting 250 LSBs from the read value. 250 LSBs equal one g when the accelerometer is programmed at 4 mg/lsb. There is a discrepancy between the sparkfun website and the ADXL345 datasheet which says that the sensitivity is actually 3.9 mg/lsb. If this is true, then one g will equal 256 LSBs. This is a small variation and should not have much of an effect on our accuracy for our application of detecting and reporting a fall. (1)

5 void calibrateaccelerometer(void) { //Take a number of readings and average them //to calculate any bias the accelerometer may have. for (int i = 0; i < 32; i++) { accelerometer.getoutput(readings); a_x += (int16_t) readings[0]; a_y += (int16_t) readings[1]; a_z += (int16_t) readings[2]; //50Hz data rate. wait(0.02); a_x /= 128; a_y /= 128; a_z /= 128; //At 4mg/LSB, 250 LSBs is 1g. a_xbias = a_x; a_ybias = a_y; a_zbias = (a_z - 250); a_x = 0; a_y = 0; a_z = 0; Figure 4: Calibration Code for ADXL345 Accelerometer Chip Project Review and Future Work: The programming for the fall detection biosensor needs to be debugged to operate more efficiently while taking up less memory space. We also need to program interrupts to bring alarm functionality to the ADXL345 breakout board. Testing of the sensor is required to record and program threshold acceleration data. The temperature sensor housing is currently a rough prototype on SolidWorks. Last week Maysarah had a design made in Google Sketchup. Further collaboration is needed to access our desired specifications and implement them to the SolidWorks program. These specifications include the spacing between the conductive fabric on the chest, the clipping mechanism, the attachment mechanism of the PCB board, and several other factors influencing the design.

6 The ECG sensor is lagging as a result to budget constraints which have been resolved. I have submitted order forms for all of the parts necessary. Upon arrival, we will assemble and test the ECG for proper functionality and accuracy. Hours Worked: Up until the submission of this weekly report, I have worked 10 hours on the fall detection sensor coding/preparation and the temperature sensor housing. I expect to work 10+ hours Thursday and Friday to get the fall detection sensor and temperature sensor housing completed. Our team will also be working over our spring break to ensure the completion of our project in a timely fashion.

Skill Level: Beginner

ADXL345 Quickstart Guide by zaggrad January 10, 2011 Skill Level: Beginner Description: The ADXL345 is a small, thin, low power, 3-axis accelerometer with high resolution (13-bit) measurement at up to